It is well known to provide electronic article surveillance (EAS) systems to prevent or deter theft of merchandise from retail establishments. In a typical system, markers designed to interact with a magnetic field placed at the store exit are secured to articles of merchandise. If a marker is brought into the field or "surveillance zone," the presence of the marker is detected and an alarm is generated.
One type of magnetic EAS system is referred to as a harmonic system because it is based on the principle that a magnetic material passing through an electromagnetic field having a selected frequency disturbs the field and produces harmonic perturbations of the selected frequency. The detection system is tuned to recognize certain harmonic frequencies and, if present, causes an alarm.
A basic problem in the design of markers for harmonic EAS systems is the need to have the marker generate a harmonic signal that is both of sufficient amplitude to be readily detectable and also is sufficiently unique so that the detection equipment can be tuned to detect only the signal generated by the marker, while disregarding harmonic disturbances caused by the presence of items such as coins, keys, and so forth. A known approach to this problem is to develop markers that produce high order harmonics with sufficient amplitude to be readily detectable. A particularly useful technique along these lines is disclosed in U.S. Pat. No. 4,660,025, issued to Humphrey, the disclosure of which is incorporated herein by reference. The Humphrey patent discloses a harmonic EAS marker employing as its active element a wire or strip of 5 magnetic material which has a magnetic hysteresis loop with a large discontinuity, known as a "Barkhausen discontinuity". Upon exposure to an alternating-magnetic field of sufficient amplitude, the active element undergoes substantially instantaneous regenerative reversals in magnetic polarity, producing very sharp signal spikes that are rich in detectable high harmonics of the frequency of the alternating field.
Markers employing the type of active element just described have been successfully placed in practice and are in widespread use with harmonic EAS systems distributed by the assignee of the present application under the trademark "AISLEKEEPER".
It has been desired to reduce the size, and particularly the length of harmonic markers which employ active elements of the type disclosed in the Humphrey patent. One constraint upon reducing the length of the active element is that large Barkhausen discontinuities can only be produced in active elements having a high ratio of length to cross-sectional area to provide a very low demagnetizing factor. It could be contemplated to reduce both the length and cross section of active elements, or to form the active elements as thin films, but the resulting elements are very low in mass, and produce signals that are too low in amplitude for reliable detection.
U.S. Pat. No. 5,519,379, which has a common assignee and a common inventor with the present application, discloses a harmonic marker which includes three lengths of wire, arranged in parallel with each other. The three wires have the above-described hysteresis loop with a large Barkhausen discontinuity. Charge spreading elements are provided at the ends of the three wires to magnetically couple the wires so that all three wires switch magnetic polarity substantially simultaneously upon exposure to the alternating magnetic field used to detect the marker. The charge spreading elements (which can also be considered flux concentrating elements) each have a magnetic anisotropy that is oriented in substantially the same direction as the three wires. The simultaneous switching of the three wires provides a signal that is comparable in amplitude and sharpness to that provided by a single, longer wire.
U.S. Pat. Nos. 4,075,618 and 4,710,754 disclose harmonic markers in which a relatively wide flux concentrating element is provided integrally at each end of a relatively narrow "switching" section which constitutes the active element of the harmonic marker.
In addition to high signal amplitude and reduced length, it is another desirable characteristic of a harmonic marker that its hysteresis loop characteristic be "stable". That is, it is desirable that the threshold level, which is the applied field level at which the Barkhausen discontinuity occurs, be substantially unchanged from cycle to cycle of the alternating interrogation field. When a marker exhibits an unstable hysteresis loop characteristic, it may be difficult to reliably detect the marker.